Method for producing concentrated nitric acid

ABSTRACT

A PROCESS FOR EFFICIENTLY PRODUCING CONCENTRATED NITRIC ACID WHICH COMPRISES THE STEPS OF (1) ABSORPTION OF NITROGEN PEROXIDE, (2) STRIPPING OF NITROGEN PEROXIDE, (3) SYNTHESIS OF CONCENTRATED NITRIC ACID FROM DILUTE NITRIC ACID, NITROGEN PEROXIDE AND AIR, AND (4) WASHING OF EXHAUST GAS, CHARACTERIZED IN THAT THE 80-90 WEIGHT PERCENT CONCENTRATED NITRIC ACID OBTAINED IN THE NITRIC ACID SYNTHESIS STEP IS USED AS AN ABSORBENT FOR NITROGEN PEROXIDE; NITROGEN PEROXIDE IN THE FUMING NITRIC ACID OBTAINED IN THE NITROGEN PEROXIDE ABSORPTION STEP AND THE CONCENTRATED NITRIC ACID OBTAINED IN THE NITRIC ACID SYNTHESIS STEP IS STRIPPED FROM CONCENTRATED NITRIC ACID WITH THE FEED AIR; THE STRIPPED NITROGEN PEROXIDE IS CHARGED TO THE NITRIC ACID SYNTHESIS STEP TOGETHER WITH THE FEED AIR; UNREACTED NITROGEN PEROXIDE CONTAINED IN THE EXHAUST GAS FROM THE NITRIC ACID SYNTHESIS STEP IS ABSORBED IN THE FEED DILUTE NITRIC ACID AND IS THEN RECYCLED TO THE NITRIC ACID SYNTHESIS STEP; AND UNABSORBED GAS IN THE EXHAUST GAS WASHING STEP IS RECYCLED TO THE NITROGEN PEROXIDE ABSORPTION STEP.

Jan. 30, 1973 TETSUYA OHRUI ETAL 3,714,333

METHOD FOR PRODUCING CONCENTRATED NITRIC ACID Filed May 18, 1971INVENTORS TETSUYA OHRUI YASUHITO SAKAKIBARA JUNJI TANAKA United StatesPatent 3,714 333 METHOD FOR PRODUClN-G CONCENTRATED NlTRiC ACE) TetsuyaOhrui, Niihama-shi, Yasuhito Sakaliiliara, Saijoshi, and .lunji Tan-aka,Niihaina-shi, Japan, assignors to Suinitomo Chemical Company, Limited.Usaka, J2 an P Filed May 18, 1971, Ser. No. 144,572 tl'laims priority,application Japan, May 27, 1970, 45/ 45,922 Int. Cl. C01b 21/40 US. Cl.423392 7 Claims ABSTRACT 9F THE DISCLOSURE A process for eificientlyproducing concentrated nitric acid which comprises the steps of (1)absorption of nitrogen peroxide, (2) stripping of nitrogen peroxide, (3)synthesis of concentrated nitric acid from dilute nitric acid, nitrogenperoxide and air, and (4) washing of exhaust gas, characterized in thatthe 80-90 weight percent concentrated nitric acid obtained in the nitricacid synthesis step is used as an absorbent for nitrogen peroxide;nitrogen peroxide in the fuming nitric acid obtained in the nitrogenperoxide absorption step and the concentrated nitric acid obtained inthe nitric acid synthesis step is stripped from concent ated nitric acidwith the feed air; the stripped nitrogen peroxide is charged to thenitric acid synthesis step together with the feed air; unreactednitrogen peroxide contained in the exhaust gas from the nitric acidsynthesis step is absorbed in the feed dilute nitric acid and is thenrecycled to the nitric acid synthesis step; and unabsorbed gas in theexhaust gas washing step is recycled to the nitrogen peroxide absorptionstep.

The present invention relates to a novel process for producingconcentrated nitric acid.

In general, nitric acid (HNO is produced by converting nitrogen monoxide(NO) obtained by the oxidation of ammonia (NH;,) with air into nitrogenperoxide (an equilibrium mixture of N0 and N O hereinafter referred tosimply as N O and reacting the thus obtained nitrogen peroxide withwater (H O). However, it is difficult to obtain nitric acid of aconcentration higher than the azeotropic composition owing to lowpartial pres sure of N 0 As is well known, in HNO -H O system, anazeotrope is formed at 68 percent 'by weight of NHO It is thereforeimpossible to concentrate a dilute nitric acid having a concentrationlower than the azeotropic composition to concentrated nitric acid by asimple distillation operation.

Various processes for the production of concentrated nitric acid haveheretofore been proposed. For example, extraction distillation processusing a dehydrating agent such as concentrated sulfuric acid ormagnesium nitrate and direct synthesis process wherein nitric acid of98% by weight concentration is produced from oxygen (0 dilute nitricacid and liquid N 0 by one step under a high pressure have been carriedout on a commercial scale.

However, the above-mentioned processes have not necessarily beensatisfactory in that auxiliary materials such as concentrated sulfuricacid and oxygen are required and consumption of utilities such as steamand electric power is large.

It is therefore the principal object of the present invention to avoidthe difliculties heretofore encountered in the prior art processes forthe production of concentrated nitric acid.

It is a further object to provide a process for the synthesis ofconcentrated nitric acid wherein no auxiliary materials are required andconsumption of utilities is small.

Another object of the present invention is to provide a process forrationally producing concentrated nitric acid having a concentration ofby weight or more from nitrogen peroxide, dilute nitric acid and air ata low cost.

These and other objects and advantages of the invention will appear fromthe following description of the invention.

More particularly, a process for the production of concentrated nitricacid according to the present invention consists of steps of:

( 1) Absorption of N 0 into concentrated nitric acid (2) Stripping of N0 with air from concentrated nitric acid containing nitrogen peroxide(3) Synthesis of concentrated nitric acid from dilute nitric acid, N 0and air, and

(4) Washing of exhaust gas.

The process of the present invention is characterized in that theconcentrated nitrc acid of 80-90% by weight obtained in the nitric acidsynthesis step is used as an absorbent for N 0 N 0 contained in thefuming nitric acid (concentrated nitric acid containing N 0 obtained inthe N 0 absorption step and the concentrated nitric acid obtained in thenitric acid synthesis step is stripped with air, which is introduced asa starting material, and the stripped N 0 is supplied to the nitric acidsynthesis step together with the air; unreacted N 0 contained in theexhaust gas from the nitric acid synthesis step is absorbed in the feeddilute nitric acid in the exhaust gas washing step and is recycled tothe nitric acid synthesis step together with the dilute nitric acid;unabsorbed gas in the exhaust gas washing step is circulated to the N 0absorption step; and these steps and apparatuses are operated atsubstantially the same pressure.

Referring now to the drawing attached hereto, there is presented aprocess flowsheet illustrating one embodiment of the present invention.

In the drawing, it will be noted that a gas containing nitrogen oxidessuch as NO and N0 is mixed with the unabsorbed gas from an exhaust gasscrubber 17 which is supplied through a conduit 20 and the mixture isthen introduced to a N 0 absorption tower 2 through a conduit 1. Here,NG contained in the nitrogen peroxide-containing gas is oxidized into N0 with oxygen contained in the unabsorbed gas from the exhaust gasscrubber. Said N 0 absorption tower 2 is a stainless steel tower havingperforated plates and a cooling coil 3. The concentrated nitric acid of8090% by weight concentration is charged into the tower 2 at its topthrough a conduit 4 and is directly contacted with the nitrogenperoxide-containing gas to absorb N 0 The concentration of N 0 absorbedin the concentrated nitric acid and the absorption eliicency of N 0depend upon the number of plates in the absorption tower and theoperating conditions, and are 10-30% by weight and 98%, respectively, ata pressure of 3 to 10 atmospheres and a temperature of 20 to 50 C. It isgenerally preferable to use the gas produced by oxidizing ammonia withair as said nitrogen peroxide-containing gas.

The nitric acid of a 80-90% by weight concentration containing dissolvedN 0 is then led to a N 0 stripping tower 6 at its top through a conduit5. The synthesized concentrated nitric acid as mentioned below issimultaneously charged to the top of the tower 6 through a conduit 9.The N 0 stripping tower 6 is a tower which contains perforated platesand a heating coil 7. In the tower 6, the feed air supplied to the towerat its bottom through a conduit 8 is contacted with the above-mentionedN 0 containing nitric acid ot strip off N 0 The stripping of N 0, in thetower 6 is usually operated at a pressure of 3 to 10 atmospheres and ata temperature of 30 to 100 C.

Thus, the 99% or more of N dissolved in nitric acid 4 As describedabove, in the present invention, absorption and stripping of N 0 arecarried out under the same pressure. Therefore, a consumption of powerrequired for the recycle of the absorbent may be small. Additionally itis is stripped off in this procedure. 5 possible to utilize rather wasteheat of low temperature The feed air is led to a nitric acid synthesisreactor 12 level as the heat source for the stripping of N 0,, becausetogether with the stripped N 0 through a conduit 11 the stripping of N 0is carried out in the presence of the and is reacted with H O in thedilute nitric acid supplied feed air and thus the stripping temperaturecan be reduced.

2 u I to the reactor 12 through a conduit 14. Thus, HNO 1s The synthesisof HNO is also carried out at almost the roduced according to a reactionrepresented by the forsame pressure as in the stripping of N 0Therefore, the P mula, stripped N 0 may be charged to the synthesisreactor N O +H O+ /zO 2HNO (1) as such together 'with the feed air.Thus, complicated The nitric acid synthesis reactor 12 is a so-calledgas f z g g such as supplement of sparged stirred tank type reactor madeof stainless steel e 4 Ion i y a g or comand has a stirrer 13 and acooling coil 15 for removing 15 p i as gp iffi may e cimme d heat ofreaction. Heat of reaction transferred into cooling b y o e e aust gas aso came at media may be, otherwise, heat-exchanged with an external 53 3an e g P i .theTahbsorptlon of recycle cooler. The concentration of theresulting nitric 2 4 e syn ass of t a l acid depends upon theconcentration of the feed dilute reabslrptm of unreeeveree 9 m (mute initric acid, the N 0 concentration of the liquid phase in 335% and atllaesfer z saldhdliute mmc acld contamlng the synthesis reactor, thepartial pressure of oxygen in the 2 f Fi 6 mtnc S i synt e818. reactormay be came synthesis gas phase and the temperature, pressure and timeout Wlt out any a dmonel eqmpment such as a Pump of the reaction. If thefeed dilute nitric acid is of a conor a compressor Further Itunnecessary 9 regenerate centration of 6070% by weight and the oxygensource or Supplement the l f and both f l i used is air, concentratednitric acid of a 80-90% by weight opegatlonserehvery slmple Smee a Partof the mtne and concentration can be easily produced at a pressure of 3to 2 s i tNeosynthesls reactor may be used as the 10 atomspheres and atemperature otf 20 to 70 C. It is a if}: enthor 2 usually advantageousfor commercial production to use a e Present mventl'on has a grefltaflvantage dilute nitric acid having a concentration of about 60-70%that eqmemems and operatfone can be slmphfied Power by Weight as astarting material although there is no par- COBSQmPtIOH ,e be eurealledPe waste heat be ticular limitation to the concentration of the feeddilute efieefively enhzed Y ranonal utlhzatlon and eombma' nitric acid.non of starting materials and apparatuses.

As the synthesized nitric acid contains a large amount The followmgexamlfle luestrates but does not 1mm of N 0 dissolved, it is charged tothe N 0 stripping tower the Scope of the Present mvenuen' 6 at topthereof through a conduit 9 and is subjected to I EXAMPLE the N 0stripping treatment with the feed air together with the turning nitricacid supplied through the conduit 5. Concentrated nitric acid of a 85%by weight was As a result, substantially colorless nitric acid of an 80-produced under such conditions (flow rate, temperature, 90% by weightconcentration is recovered through a con- 40 pressure, nitric acidconcentration, composition, etc.) as duit 10. A part of the resultingnitric acid is recycled to the shown in Table 1 according to the processflowsheet of N 0 absorption tower 2 through the conduit 4 to be usedFIG. 1 attached hereto.

TABLE 1 Temper- Concentration and composition Flow rate, ature PressureUnit of Symbol number kg.-m0le/hr. KgJhr. C.) kgJem. (G) N0 N02 N20; 07N2 H20 HNOa percentage 25 6.0 5.5 3.4 85.0 Mole percent. 00 6.5 21.079.0 Do. 25 6.0 11.2 84.5 Do. 15 5.7 2.5 97.3 Do. 10 15.0 84.7 Weightpercent. 20 12.8 72.7 D0. 11.3 63.6 Do. 25 15.0 84.7 Do. 25 32.0 68.0Do.

Represented as N02 although is a. mixture of N 02+N202 in fact.

as an absorbent for N 0 and the remainder Off the acid is withdrawn as aproduct through the conduit 10. The product may he used for variouspurposes as such or after further concentration by distillation.

The exhaust gas from the nitric acid synthesis reactor 12 is led to theexhaust gas scrubber 17 at its bottom through a conduit 16. The scrubber17 is a stainless steel tower having perforated plates and a coolingcoil 19. Unreacted N 04 entrained in the exhaust gas is absorbed in thefeed dilute nitric acid for the synthesis of concentrated nitric acidsupplied to the top of the scrubber through a conduit 18, and is thenrecycled to the nitric acid synthesis reactor 12 through the conduit 14.The operating conditions in the exhaust gas scrubber 17 are a pressureof 3 to 10 atmospheres and a temperature of 0 to 40 C. Thus, 80 to 90%of N 0 entrained in the air which pass through the nitric acid synthesisreactor is absorbed in the feed dilute nitric acid.

The unabsorbed gas from the exhaust gas scrubber 17 still contains asmall amount of N 0 Therefore, the unabsorbed gas is charged to the N 0absorption tower 2 through the conduit 20 to recover the unabsorbed N 0What is claimed is:

1. A method for producing concentrated nitric acid comprising the stepsof (1) reacting dilute nitric acid, nitrogen peroxide and air at apressure of 3 to 10 atmospheres and a temperature of 20 to 70 C. toprdduce a concentrated nitric acid of a -90% weight concentrationcontaining nitrogen peroxide, -(2) allowing nitrogen peroxide-containinggas to be absorbed in product concentrated nitric acid of a 80-90% byWeight concentration to obtain nitrogen peroxide-containing nitric acid,(3) introducing said concentrated nitric acid of a 8090% by weightconcentration and said nitrogen peroxide-containing nitric acid into astripping step under pressure to strip nitrogen peroxide by contactingcountercurrently with the feed air and (4) allowing the gaseous nitrogenperoxide withdrawn from the nitric acid synthesis step (1) to beabsorbed in the feed dilute nitric acid at an exhaust gas scrubber,characterized in that a part of the product obtained in the strippingstep (3) is recycled to the nitrogen peroxide absorption step (2) to beused as said nitric acid for absorbing nitrogen peroxide while theremainder of the product is withdrawn as concentrated nitric acid of a80-90% by weight concentration and that the nitrogen peroxide-containingair withdrawn from the stripping step (3) and the nitrogenperoxidecontaining dilute nitric acid obtained in the exhaust gasscrubbing step (4) are introduced into the nitric acid synthesis step(1).

2. A method according to claim 1, wherein the unabsorbed gas from saidexhaust gas scrubber is introduced into the nitrogen peroxide absorptionstep and nitrogen monoxide contained in the nitrogen oxides-containinggas is oxidized into nitrogen peroxide with oxygen contained in saidunabsorbed gas.

3. A method according to claim 1, wherein said nitrogenperoxidecontaining gas used is produced by oxidizing ammonia with air.

4. A method according to claim 1, wherein said dilute nitric acid is ofa concentration of 60-70% by weight.

5. A method according to claim 1, wherein the absorption of nitrogenperoxide is operated at a pressure of from 3 to 10 atmospheres and at atemperature of from 20 to 50 C.

References Cited UNITED STATES PATENTS 3,542,510 11/1970 Newman et al.23-160 R 3,634,031 1/ 1972 Hellmer et a1 23-160 R 2,028,402 1/ 1936Luscher 23160 R GEORGE O. PETERS, Primary Examiner US. Cl. X.R. 423394

